Tesco and the oil company Esso also cut their prices. The three control prices at 1,200 of Britain's 10,000 service stations.
Analysts said that BP, Shell and the other oil giants were likely to drop their prices in days, providing further relief to drivers faced with record prices.
The cuts follow a fall in the cost of crude oil, which slipped 1 per cent to $64 a barrel yesterday amid signs that supplies were recovering and the recent price rise had reduced demand.
At a meeting on Monday, members of Organisation of Petroleum Exporting Countries (Opec) will discuss whether to increase maximum production by 2 per cent, or 500,000 barrels a day. Drivers of about 100 lorries and coaches in south Wales had staged a go-slow convoy from Llanelli to Newport.
Before they set off at 7.30am, police handed each driver a warning under the 1986 Public Order Act that they must travel at at least 40mph to reduce congestion. Predictions by the protesters of 50-mile delays proved wide of the mark.
But the lead lorries of the dozens of vehicles remaining slowed to 15mph by the time the convoy reached the M4, causing a four-mile tailback across all three lanes of the motorway. Police said that they would be speaking to the drivers.
Mark Greene, a member of yesterday's convoy, said: "We are quite happy with the way that things have gone today. What has happened today is just showing what we can do."
Tom Lavender, a taxi driver from Newport who joined the protests, said: "We have got to keep on and drive home the message that fuel duty is too high."
Petrol pump prices for super unleaded passed the £1 mark last week after the crude oil price jumped because of damage from Hurricane Katrina and demand from China. Threats of widespread protests this week caused a bout of panic-buying of petrol on Tuesday that led to supplies running out at about a third of the country's garages. But Wednesday's planned day of action attracted only a few dozen protesters to oil refineries.
Last night Asda urged rivals to follow its lead of putting its maximum price on its website. Tony Page, the store's general merchandise director, said: "Drivers have been ripped off in the past 10 days as they rushed to fill up. We've been shocked at some of the prices we've seen."
A Tesco spokeswoman said: "The price of petrol has fallen and we are passing those savings back on as soon as we can."
The price cuts were welcomed by the Treasury, which said it had "made clear" that prices should fall promptly in line with the international oil price.
But Ray Holloway, of the Petrol Retailers' Association, said small independent traders faced losing out once again in the price squeeze. "People should look at the number of local petrol stations closing. We think hundreds more will close this year. Do customers think supermarkets will still offer low prices when there is no competition?" The AA Motoring Trust said drivers were spending £7.5m more a day on petrol than they had at the start of the year.
Ruth Bridger, a petrol analyst for the trust, said: "Four pence off a litre is very welcome news but drivers are still dismayed by what they are paying at the pump."